Frequency variation

After amplification both signals change their initial phases due to the delay r of the amplifier unblank (r = 0.1 - 0.5 ms), phase shift in it and wave propagation in passive vibrator s elements. All the mentioned phase changes are proportional to the frequency. The most contribution of them has unblank delay z. Thus frequency variations changes the initial phases) f/, and j(/c) of both signals and their difference A - Vi ... 

The kinetic nature of the glass transition should be clear from the last chapter, where we first identified this transition by a change in the mechanical properties of a sample in very rapid deformations. In that chapter we concluded that molecular motion could simply not keep up with these high-frequency deformations. The complementarity between time and temperature enters the picture in this way. At lower temperatures the motion of molecules becomes more sluggish and equivalent effects on mechanical properties are produced by cooling as by frequency variations. We shall return to an examination of this time-temperature equivalency in Sec. 4.10. First, however, it will be profitable to consider the possibility of a thermodynamic description of the transition which occurs at Tg. 

Voltage unbalance System harmonics Voltage variation Frequency variation Ambient temperature and Altitude. 

Table 1.4 Combined permissible voltage and frequency variations...

Note IS 325 specifies voltage variation 6% and frequency variation 3% or any comhination of these. 

On a 3-0 system the frequency variation is normally within limits, which according to lEC 60034-1 is 2% (Figure 1.6). The reason for keeping the frequency variation low is that it is not influenced by any condition outside the generating point, and at the generating point, it is maintained constant through automatic speed regulation of the prime mover (frequency is directly proportional to the speed, equation (1.6a)). The effect of frequency variation, however small, is discussed below ... 

Table 1.6 Approximate effects of voltage and frequency variations on motor performance...

Note The speed of the motor ean be varied by vai-ying the frequeney alone but this docs not provide satisfactory performance, A variation in frequeney causes an inverse variation in the flux, for the same system voltage. The strength of magnetic field, p, develops, the torque and moves the rotor, but at lower speeds. / would be reduced, which would raise 0 , and lead the magnetic circuit to saturation. For higher s )ccds, / would be r.nised, but that would reduee which would adversely diminish the torque. Hence frequency variation alone is not recommended practice for speed control. The recommended practiee is to keep V/fas constant, to maintain the motor s vital operatin.c parameters, i.e. its torque and 0 ,. within acceptable limits. 

By varying the gale firing angle, V can also be varied. With SCRs frequency variation i.s not possible. 

Note Since an inverter is not line commutated, the SCRs have a sw-iiching limitation and hence a limitation in frequency variation. When lliyrislors ai e to perform frcquenl switchings, CTOs are used In the inverter circuit. 

Induction generator This is a standard squirrel cage motor with additional treatment to weather the site conditions. The normal specifications are generally the same as for a standard motor. The permissible voltage and frequency variations are, however, wider as noted below ... 

Combined voltage and frequency variation not to exceed 10%. Voltage may fall to 80% during running for about 20... 

The governor can be set to make the machine run at a constant speed, even on load variations, with extremely quick and almost instantaneous speed control, and thus maintain a near-zero AN. In a parallel operation they can also control load sharing automatically and accurately. Power grids, receiving power from different sources, are extremely susceptible to frequency variations. Even a small Af of the order of 0.5 Hz, may cause the system to trip. A fast-actuating governor with low response time (as low as 0.5 second) can overcome such a situation by quickly regulating the speed of the PM. 

Apart from considerations given to corrosion resistance and suitability for hazardous (classified) areas, the selection of electric motors for oil field applications is the same as the selection of electric motors for other industrial applications. One exception may be the selection of motors for areas where electric power is self-generated. Frequency and voltage variations may occasionally occur at such locations. For such locations, consideration should be given to specifying motors which are tolerant to at least 10% voltage variations and 5% frequency variations. 

Figure 13-18. Pulse energy ratio for frequency variations.

A.c. impedance. Measurements of the frequency variation of impedance allow separation of the change transfer resistance from the contributions to the total impedance of the environment resistance, surface films, adsorbed layers, etc. Robust instruments utilising a two-frequency technique have been developed . 

RF frequency variation. In Figure 18 are shown the effects of RF frequency on the partial pressures of silane, hydrogen, and disilane (Fig. 18a) and on the deposition rate (Fig. 18b). The total pressure is 16 Pa, and the plasma power is 5 W. The discharge is in the a-regime. 

Baker A, Smart PL and Ford DC (1993a) Northwest European palaeoclimate as indicated by growth frequency variations of secondary calcite deposits. Palaeogeog Palaeoclim Palaeoecol 100 291-301 Baker A, Smart PL Edwards RL (1995) Paleoclimate implications of mass spectrometric dating of a British flowstone. Geology 23 309-312... 

Mow to calculate the transition probability in the ease in which all frequencies v — o>/2n are incident on the molecule, Eq. (87) must be integrated over the frequency range. As the significant frequency variation is due to the factor... 

E. Louie, J. Ott, and J. Majewski, Nucleotide frequency variation across human genes. Genome Res. 13, 2594-2601 (2003). 

Air preheaters employed in power generating plants are either the tubular or regenerative types. Both are periodically washed to remove deposits that accumulate. The frequency of washing is typically once per month however, frequency variations ranging from 5 to 180 washings per year are reported. Many preheaters are sectionalized so that heat transfer areas may be isolated and washed without shutdown of the entire unit. 

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